Control method and control device for internal combustion engine

A control device of an internal combustion engine comprising:

• a three-way catalyst provided in an exhaust passage of the internal combustion engine;
• an upstream air fuel ratio sensor and a downstream air fuel ratio sensor provided at an upstream side and a downstream side of the three-way catalyst, respectively;
• and
• a controller configured to control a fuel injection amount, wherein the controller is configured to diagnose catalyst deterioration based on decrease in an oxygen storage capacity of the three-way catalyst from a reference oxygen storage capacity, wherein the controller is configured to set the reference oxygen storage capacity according to a flow amount of gas flowing into the three-way catalyst so that as this gas flow amount increases, the reference oxygen storage capacity decreases, estimate an oxygen storage amount of the three-way catalyst based on the gas flowing into the three-way catalyst, and when an air fuel ratio at a downstream side of the three-way catalyst is changed to lean, reset an estimated value of the oxygen storage amount using the reference oxygen storage capacity set by taking the gas flow amount into consideration.